The warnings have come from the White House, Western capitals and the U.N. Syria's president, Bashar al Assad, must not use chemical weapons against the rebels and his people.

Publicly, Syrian officials deny having a chemical stockpile. They insist they would never use one if they had one. But U.S. officials have said there are signs that the Syrian army is readying its chemical arsenal for use.

NPR's Deborah Amos recently met with the former head of Syria's chemical weapons division, who defected and is now with the rebels. And she joins us now from Antakya, Turkey.

Good morning.

DEBORAH AMOS, BYLINE: Good morning.

MONTAGNE: Now, Syria's known to have the largest undeclared caches of chemical weapons in the world. Confirming where they are and what's being done has been a significant challenge. What have you learned?

AMOS: Well, I interviewed Major General Adnan Sillu. He's now here in southern Turkey. He said that he was sure that Bashar al Assad would use chemical weapons if he was desperate enough, and he confirmed what's long been reported, that Syria does have the largest stockpile of chemical weapons in the world. These are considered weapons of mass destruction; they're banned by international treaty. Syria has tons of this stuff and Sillu listed the specifics.

SILLU: (Through translator) Yes. This is what we have in the chemical arsenal in Syria.

MONTAGNE: And this is a man who would know. Why does Syria have such a large arsenal?

AMOS: Renee, the stockpile was built up over decades and he said it was a strategic deterrent against Israel. He said the chain of command is very clear. President Bashar al-Assad is the only one who can order the use of chemical weapons. Those orders would be carried out by the chief of air force intelligence and Assad's top security advisor.

Sillu also told me that the chemical arsenal is stored in warehouses around the country, some near military airports. He said there's a research center near Aleppo, in northern Syria, where Iranians worked with Syrian scientists. He said the Iranians were always coming to visit and advise.

Now, there's been fierce fighting in the north. So there's some speculation that the Syrian government moved the chemical arsenal to keep it out of the hands of the rebels.

MONTAGNE: And are rebels worried that Assad will use the chemical weapons to stop advances on the capital?

AMOS: Sillu said there was some concern. He was in charge of training Syrian troops to protect themselves in the event of chemical warfare. He said that gas masks are standard issue. He said so far the rebels aren't interested in capturing the weapons, but they are trying to capture the warehouses where the gas masks are stored. And here's what he said.

SILLU: (Through translator) Most of the rebels are defected from the Syrian army and they know how to protect themselves. And he told the rebels, wherever you find the gas masks, grab it.

AMOS: Have the rebels collected gas masks?

SILLU: (Through translator) No, they don't have now, but very soon they will take control over this storage.

AMOS: That's Major General Adnan Sillu, the former head of Syria's chemical weapons program. He said he wants to create a force to secure those weapons, but he's had no takers from Western governments.

MONTAGNE: What would it take to secure those warehouses?

AMOS: Well, earlier this year a Pentagon report concluded it would take 70,000 troops to find and secure those warehouses. There are unconfirmed reports that U.S. contractors are training Syrian Army defectors in Jordan as a chemical strike force. In NATO it's the Czech military that has specialists in chemical weapons, and they are reportedly in Jordan.

Chemical weapons are a serious threat. Major General Sillu is convinced that Bashar al Assad will eventually use them if the rebels close in on Damascus, a last desperate act. But there are other Syrian analysts that say it's regime suicide. Western governments, including the United States, have threatened intervention if Assad uses those weapons.